1. Technical Field
The present invention relates to frequency measurement devices and, more particularly, to improvements of a frequency measurement device that is capable of measuring an absolute frequency, using a frequency measurement device that counts a signal to be measured for a predetermined time, and removes high frequency components from a stream of counted values thereby detecting a frequency change component.
2. Related Art
As frequency measurement methods, a direct count method that counts pulses passing within a predetermined gate time (see, for example, JP-A-2001-119291 (Patent Document 1)), a reciprocal method that accurately measures a pulse period and obtains a frequency from its time reciprocal (see, for example, JP-A-05-172861 (Patent Document 2)), and a method that obtains a delta sigma (ΔΣ) modulation signal to acquire a frequency (see, for example, U.S. Pat. No. 7,230,458 (Patent Document 3)) are known.
The inventors of the invention have proposed a “short-gate time count method” (see JP-A-2009-250807) as a novel frequency measurement method having characteristics that are nonexistent in the methods of related art. According to the short-gate time count method, counting (sampling) of pulses is continuously repeated with a short-gate time, and high frequency components are removed from a stream of the obtained counted values, whereby an output corresponding to the frequency of the signal to be measured is obtained. By this method, both of the time resolving power and the frequency resolving power can be considerably improved, compared to the methods of related art.
The frequency counter of the proposed method uses a short-gate counter section and a low-pass filter section that removes high frequency components. When a digital filter is used as a part of the composition of the low-pass filter, for example, the use of a moving average filter is proposed. The use of a moving average filter can substantially reduce the amount of computation, enabling highly accurate real-time measurement.
The short-gate counter section has a relatively simple hardware structure, and therefore is suitable for high-speed operation. However, the processing at the filter section needs addition and subtraction of multiple bits, such that the upper limit of sampling frequency in real time measurement is defined, mainly, by the processing power of the filter section. To obtain a moving average filter with desired characteristics, it is necessary to structure a shift register with multiple bits in multiple stages and to operate such a shift register at high speed, resulting in a greater circuit scale and greater power consumption.